DE2902706A1 - PROCEDURE FOR CONTROLLING AMMONIA INJECTION IN A DRY PROCESS FOR DENITRATING EXHAUST GAS - Google Patents

Description

Henkel, Kern, Feiler & Häiuel patent attorneys

Registered Representatives before the J European Patent Office

Mitsubishi Jukogyo Kabushiki Kaisha,

Möhlstrasse 37 Tokyo, Japan D-8000Munich80

Tel .: 089 / 982085-87

Telex: 0529802 hnkl d telegrams: ellipsoid

lh. Jan. 1979

2298

Process for regulating the ammonia injection in a dry process for exhaust gas denitration

description

The invention relates to a method for regulating the injection of ammonia in a dry process for exhaust gas denitration.

As a dry process for exhaust gas denitration, a catalytic denitration process is currently widely used, in which nitrogen oxides (NO) in a combustion _{exhaust gas and ammonia (NH 3} ), which is blown into the system from the outside and mixed with the exhaust gas, undergo a catalytic reaction with an activator subjected to a catalyst surface and decomposed into harmless nitrogen and water. In addition, a non-catalytic denitration process in which NO and NH- undergo a vapor phase reaction in a high temperature range with decomposition is currently being developed for use.

However, most of the NO contained in a combustion exhaust gas is present as NO, the proportion of which depends on the

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Temperature conditions such as combustion temperature or the like varies to some extent. In each of the denitration processes mentioned, however, a process takes place in which an equivalent reaction between NO and NH ₃ , ie the following reaction:

4N0 + 4NH ₃ + O ₂ 4N ₂ + 6H ₃ O

is considered to be the main reaction, with NO being decomposed and eliminated by automatically blowing in NH ₃ in an amount corresponding to the NO equivalent to be decomposed or a slightly larger amount of NO.

In this previous method for controlling the NH ₃ -EXnblasung the amount of NO contained in the exhaust gas is measured or stored by the NO concentration is multiplied by the exhaust gas flow rate, and the amount of NH _{3 to be} injected is determined by multiplying the measurement signal with adjusted to an intended NH ₃ / NO ratio. Since this ratio is set or determined manually, this ratio remains constant. The previous procedure is therefore fraught with the following deficiencies:

1. Although the NO concentration is measured by means of an automatic analyzer, there is a delay of a minute or more before the measurement results are converted into transmission signals and before they are output, mainly due to a delay in the analyzer system, for example due to a gas exchange in a sample line, so that a considerable delay is introduced in the tracking of the _{amount of NH 3} introduced and, as a result, the denitration performance cannot follow a rapid change.

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2. The rate of the denitration reaction between NO and NH- fluctuates depending on the temperature, and the temperature dependency of the denitration performance varies depending on the type of catalyst, so that a flat denitration performance curve cannot always be obtained _{with a constant NH 3 / NO ratio.}

3. If the exhaust gas temperature drops (eg to 300 ⁰ C or below), the catalyst is poisoned due to the absorption of NH ₃ on the catalyst surface and due to the ammonium salt formed by the reaction between NO in the exhaust gas and NH- so that it becomes necessary to reduce the NH / NO ratio; In the previous process, the introduction of NH ₃ was therefore ended in a low temperature range in order to interrupt the denitration process.

4. When the temperature drops, the amount of NH ₃ absorbed on the catalyst surface increases, while when the temperature rises, the NH ₃ absorbed at low temperature is released and dispersed in the exhaust gas, so that the NH / NO ratio in the exhaust gas increases

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and »NH ₃ emerges with the gas at the outlet of the denitration device. This not only has unfavorable effects on associated devices connected downstream of this outlet (for example an air heater), but rather this escaped NH _{3 can} possibly become a significant cause of secondary environmental pollution.

The object of the invention is to eliminate the described shortcomings and disadvantages of the previous methods by creating an improved method for regulating the blowing in or introduction of NH ₃ in a method of the type mentioned while regulating the NH / NO ratio.

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This object is achieved according to the invention in a method for regulating the ammonia injection in a dry method for exhaust gas denitration, in which ammonia is blown into a combustion exhaust gas containing nitrogen oxides and mixed with the exhaust gas in order to decompose and thus eliminate the nitrogen oxides, according to the invention that at least a first processor unit is used in which, based on a relationship between a given nitrogen oxide concentration and a numerical variable for a combustion value, such as fuel throughput, combustion air throughput, exhaust gas throughput, feed water throughput or throughput of the generated steam, initially or provisionally the amount of nitrogen oxides as a function of this numerical variable is derived and stored that a second processor unit is used, in which an ammonia / nitrogen oxide ratio as a function on the basis of the relationship between the exhaust gas temperature and the denitration performance ion of the exhaust gas temperature is initially or provisionally derived and stored, and that the optimal amount of ammonia to be injected is determined by the fact that the amount of nitrogen oxide, which has been determined by entering the numerical variable for the combustion variable in the first processor unit, with an ammonia / nitrogen oxide Ratio is multiplied _? which has been determined by entering the exhaust gas temperature or a numerical variable related to this into the second processor unit.

The invention is characterized in a modified embodiment in that a third processor unit is provided in which, on the basis of a relationship between a time change amount or rate of the exhaust gas temperature and the ammonia / nitrogen oxide ratio a characteristic or Characteristic curve factor for correcting the above-mentioned ratio as a function of the amount of time change is provisionally derived and is stored, and that the multiplication is carried out with correction by the third processor unit.

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In the following, preferred exemplary embodiments of the invention are explained in more detail with reference to the accompanying drawings. It demonstrate:

1A and 1B are flow charts illustrating a preferred embodiment of the invention Procedure,

FIG. 2 is a graph showing the relationship between the exhaust gas temperature and the exhaust gas flow rate, and FIG the degree of denitration,

Fig. 3 is a graphical representation of the dependence of

exiting NH ₃ amount depends on the exhaust gas temperature and the NH ₃ / NO ratio and

4 shows a flow diagram to illustrate another exemplary embodiment of the method according to the invention.

According to FIGS. 1A and 1B, the latter of which corresponds to the former, but includes a schematic representation of the combustion system, a fuel A becomes a burner device B, such as a sintering furnace, a furnace or the like, supplied. This creates a in the device B. NO -containing exhaust gas C ', which in a smoke pipe C with over NH- fed to a line 14, is mixed to add NO

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reduce and decompose. The mixture is then fed to a denitration device D, in which the gas mixture is brought into contact with a denitration catalyst having a granular, honeycomb, tubular or plate-like shape, and NO is decomposed into nitrogen and water. The gas is then discharged as treated exhaust gas E ¹ via an air preheater H, a dust collector K and a suction fan J through a smoke pipe E from the system.

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In the case of the denitration device described, the denitration performance depends on the temperature characteristic of the Catalyst effectiveness from, so that the reaction rate between NO and NH is not constant. For example, if the NH, / NO ratio is constant, results in the Relationship between the exhaust gas temperature at the catalytic converter on the one hand and the denitration rate and the exhaust gas flow rate on the other hand from Fig. 2.

The solid line a in Fig. 2 illustrates the relationship between the exhaust gas temperature and the size or degree of denitration with a constant exhaust gas flow rate. How out As can be seen from this curve, the degree of denitration improves with increasing exhaust gas temperature. With increasing Exhaust gas temperature, i.e. under high load conditions, increases however, the exhaust gas flow rate in the manner indicated by the dash-dotted line b, with increasing Exhaust gas flow rate at constant exhaust gas temperature of the The degree of denitration generally decreases (not shown). In the case of a practical relationship between exhaust gas temperature and denitration ratio, therefore, results in a low denitration degree at high temperature and at low temperature, as indicated by the dashed line c, while the degree of denitration is otherwise a has a substantially flat temperature dependence curve.

In the method according to the invention, an NH, / NO ratio is therefore provisionally derived as a function of the exhaust gas temperature on the basis of the relationship between the temperature of the exhaust gas C ¹ and the denitration performance, so that the NH, / NO - The ratio can be increased to prevent a decrease in the degree of denitration, while in a light load and low temperature area this ratio can be decreased,

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to reduce the amount of absorption of NH ₃ on the catalyst. The derived NEW / NO ratio is stored as a function of the exhaust gas temperature in a processor (II) denoted by 4 in FIG. 1A.

Then, an output signal 1-1 of a flow meter 1 becomes for fuel A or a water supply or steam throughput quantity signal 1-2 to a function generator denoted by 2 in FIG. 1A or processor (I) entered, in which the product of the flow rate of the exhaust gas C 'and the NO concentration, i.e. the amount of NO, provisionally as a function of the combustion conditions (i.e. fuel throughput, Combustion air flow rate, etc.) is stored and an NO amount signal 2-1 is generated and sent to a multiplier circuit 10 is delivered. On the other hand, an output signal 3-1 from a temperature sensor or detector 3 for the exhaust gas C converted by the temperature characteristic processor 4 (processor (II)) into a NEW / NO characteristic signal 4-1. This signal 4-1 is transmitted, if necessary, to a processor 5, through which it is connected to a desired or Sol-NHo / NO ratio signal G-1 for continuous operation multi-

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This ratio can be entered manually on the handheld controller G and used if an arbitrary nominal denitration value is to be set.) An NH ₃ / N0 ratio signal 5 -1 derived, which is then passed to the multiplier circuit 10; _{the NH 3} / NO characteristic curve signal 4-1 can optionally be passed directly to the multiplier circuit 10. In the latter, the NO amount signal 2-1 is multiplied by the NH, / NO characteristic curve signal 4-1 or the NH./N0 ratio signal 5-1, so that an NH ₃ supply setting signal 10-1 is obtained. This signal 10-1 is transmitted to an NH ₃ blow-in quantity regulator 11, which controls a control valve. The NH ₃ or ammonia flow rate F is also measured by a flow meter 12, and the measuring

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signal is used for control or regulation purposes as a ΝΗ -, - injection volume regulator 11 transmitted.

Said temperature signal 3-1 for the exhaust gas C can through a signal of a numerical variable related to the exhaust gas temperature can be replaced, for example a numerical one Quantity that indicates the size of the combustion, such as fuel throughput, combustion air throughput or Exhaust gas throughput of an exhaust gas source or feed water throughput or throughput of the generated steam in the case a boiler (signal 1-2 according to Fig. 1A).

In addition, when the temperature of the exhaust gas C varies, the amount of NH ₃ absorbed on the catalyst changes, with absorption and release phenomena occurring. However, when the temperature rises, a balanced amount of absorption is decreased, and when the temperature rises suddenly, the absorbed NH _{3 is} temporarily released and dispersed (in the exhaust gas) (the release and dispersion rate being proportional to the amount of time change in exhaust gas temperature), whereby the The NH ₃ concentration in the flue pipe E increases and thus the escaping NH ₃ amount ζ un immt.

This phenomenon is explained below with reference to FIG. If the flue gas temperature changes with time in the manner shown by the solid curve d, the NH ₃ concentration in the flue pipe E shows a temporary peak (solid curve d-2) at a constant NH - / NO ratio (solid curve e) . Obviously, a _{loss of NH 3} occurs, which has an unfavorable influence on the devices downstream of the flue pipe E.

According to the invention, therefore, in the processor (III), i.e. in the Temperature curve processor 4, an NH., / NO ratio on

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set and saved so that this can be

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Temperature can have a small size in order _{to reduce the amount of NH 3} absorption on the catalyst. As shown by the dashed curve g-1 in FIG. 3, the NH ₃ / NO ratio is reduced at a low temperature, so that the amount of NH 4 absorbed on the catalyst can be reduced.

In another embodiment of the invention shown in FIG. 4, a temperature change quantity processor 8 (processor (III)) is provided in order to prevent NH ^ leakage when the temperature rises, in which, on the basis of a relationship between a time change quantity, the temperature of the exhaust gas G ¹ and an NH ₃ / NO _x ratio, a characteristic factor for correcting the NH -, / NO ratio as a function of the temperature-time change variable is derived and stored, with a temperature signal 3-1 being supplied to said processor 8, which is a negative signal for reducing the NH ₃ / NO _x ratio in the event of a temperature rise (cf. solid curve d-2 in FIG. 3) generated as an NH ₃ / NO ratio correction signal 8-1. This signal 8-1 is fed to a multiplier circuit 9, in which the signal 8-1 with the NH -> / NO characteristic curve signal 4-1 or the NH ₃ / NO _x ratio signal 5-1 from the mentioned temperature characteristic processor 4 or from the processor 5 is multiplied. The product signal is then transmitted to the multiplier circuit 10 as a corrected NH-3 / NO ratio signal 9-1. The signal flow paths following the multiplier circuit 10 correspond to those according to FIG. 1.

The mode of operation of the exemplary embodiment described above is explained below with reference to FIG. At low temperatures, the ΝΗ, / ΝΟ ratio has, as mentioned, somewhat lower values (cf. curve g-1), so that the NH ₃ concentration in the flue pipe E is also slightly lower than that of the solid curve f (see dashed curve h-1) remains. If the temperature rises (solid curve d-2), the temperature change output

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size negative, with temporarily low values of the NEW / NO ratio (cf. dashed curve g-2), so that even if the NHg released by the catalyst is added, the NH ₃ concentration in the flue pipe E, as indicated by the dashed line h -2 shown, no peak value is reached and thus the NH ₃ leakage or leakage amount can be reduced in a stable manner.

The exemplary embodiments of the method according to the invention according to FIGS. 1 and 4 can also be modified in such a way that the NH ₃ concentration in the treated gas E ¹ is measured by an analyzer 6 and the measurement signal is transmitted to an NH ₃ concentration regulator 7 in which a Deviation of this signal from an NH ₃ setpoint, which is transmitted as a further input signal 7-0 to the controller 7, is _{derived and output as an NH 3} concentration control signal 7-1, and either the output signal 7-1 is transmitted to the mentioned manual controller G and to to this to correct the desired NH ₃ / NO ratio to the signal representing the desired or Sol-NHo / NO ratio in continuous operation.

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is added or subtracted from it, or the output signal 7-1 is transmitted directly to processor 5, in which it is used to correct the NH -, / NO -Kennien lines Signa Is 4-1

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is added to or subtracted from this. In the case of this modification, this ratio can be corrected and the NH ₃ -LeCk- or - The discharge amount can be kept to a minimum.

It should be noted that the inventive method not limited to the catalytic denitration described, but equally to a non-catalytic one Denitration is applicable. In the latter case, the

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Temperature signal (corresponding to signal 3-1 according to FIGS. 1 and 4) the measured exhaust gas temperature of the reaction between NH ^ and NO in the burner device (device B according to FIG. 1 and 4) used, in a processor (II) (processor 4 according to FIG. 1) first of all an NH., / NO ratio as a function

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the exhaust gas temperature is derived and stored based on a relationship between the exhaust gas temperature the reaction between NH-. and NO and the Denitration performance. Then, similar to the previous catalytic deniting process, an optimal one becomes

NH., / NO ratio set, with NH, via a line Jx j

15 according to FIGS. 1 and 4 is blown into the burner device.

The effects or advantages of the described method according to the invention can be summarized as follows:

1. As the derivation or measurement of the NO _x content in the exhaust gas

is carried out in such a way that a NO amount signal by entering a signal for fuel throughput, exhaust gas throughput, Feed water throughput or steam delivery quantity is generated in the processor (I), in which the relationship between , these throughput quantities and the NO quantity provisionally

or initially stored and the generated NO quantity signal as a signal for regulating the amount of NH blown in, is used, the response behavior is faster than in the case of the output signal of the mentioned under 1. NO analyzer in the previous control method, so that the denitration performance is also able to follow a sudden change.

2. Since a processor (II) is used, which is based on the relationship between the exhaust gas temperature and the denitration performance the activity-temperature characteristic of a catalyst in a catalytic denitration process or the reaction temperature curve for a non-

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catalytic denitration process taken into account, and the NHo / NO ratio initially or provisionally as a function the exhaust gas temperature is stored, an optimal NEW / NO ratio signal can always be delivered, whereby a flat characteristic curve of the degree of denitration is achieved (improvement over the initially under 2. mentioned disadvantage of the previous procedure).

3. Since an optimal NH - / NO ratio can also be set in the case of a large variation in the exhaust gas temperature, if the said ratio is temporarily or initially set and stored in the processor (II) in such a way that the size of this ratio can be reduced at low temperature in order to reduce the _{amount of NH 3} absorption on the catalyst and thereby prevent catalyst contamination, the need to interrupt the denitration by terminating the NH ₃ injection, as explained at the beginning under 3, can be avoided, so that continuous denitration becomes possible.

4. Since the NH ₃ leakage or leakage quantity can be reduced by arranging the processor (III) which is able to correct the NH / NO ratio according to a measured time change of the exhaust gas temperature, the NH _{3 can} be used effectively . In addition, secondary environmental pollution and unfavorable influences on the devices downstream of the denitration device are prevented (improvement of the disadvantage of the previous method mentioned at the beginning under 4).

5. An arbitrarily selected desired or sol denitration size can be achieved by providing the NH./NO ratio transmitter or selector, which is known per se, and the desired NH ₃ / NO ratio in continuous operation

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indicating signal that is arbitrarily entered into this selector with one of the said processor (II) supplied ΝΗ, / ΝΟ ratio signal is multiplied.

6. By additionally providing a circuit in which, after deriving a deviation signal with respect to an NH-j setpoint value from a _{signal indicating the NH 3} concentration in the treated gas, the deviation signal is fed back _{as an NH 3 concentration signal, too} If the combustion conditions have changed with a change in the denitration performance and consequently the optimal NH ₃ / NO ratio has deviated, appropriate corrections can be made and the _{amount of NH 3 released} can be kept to a minimum.

7. The control of the NH ₃ injection can easily be carried out even in a denitration apparatus in which a non-catalytic and a catalytic denitration process are carried out in combination with one another.

Although the invention has been explained above in preferred exemplary embodiments, these are self-evident to the person skilled in the art various changes and modifications are possible without departing from the scope of the invention.

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Claims (2)

Henkel, Kern, Feiler & Häiu & I Patentanwälte Registered Representatives before the European Patent Office Mitsubishi Jukogyo Kabushiki Kaisha, Möhlstrasse 37 Tokyo, Japan D-8000 Munich 80 Tel .: 0 89/98 20 85-87 Telex: 0529802 hnkl d Telegrams: ellipsoid 2298 Jan. 24, 1979 Process for regulating the ammonia injection in a dry process for exhaust gas denitration Claims 1. Method for regulating the ammonia injection in a Dry process for exhaust gas denitration, in which ammonia is converted into a combustion exhaust gas containing nitrogen oxides is blown in and mixed with the exhaust gas in order to decompose and thus eliminate the nitrogen oxides, thereby characterized in that at least one first processor unit is used in which on the basis of a relationship between a given nitrogen oxide concentration and a numerical value for a combustion value, such as fuel throughput, combustion air throughput, exhaust gas throughput, feed water throughput or Flow rate of the generated steam, initially or provisionally the amount of nitrogen oxides as a function of this numerical Size is derived and stored that a second processor unit is used in which on the Based on the relationship between the exhaust gas temperature and the denitration performance, an ammonia / nitrogen oxide ratio as a function of the exhaust gas temperature initially or provisionally 909830/0867 is derived and stored, and that the optimal amount of ammonia to be injected is determined in that the amount of nitrogen oxide obtained by entering the numerical size for the combustion size in the first processor unit has been determined, is multiplied by an ammonia / nitrogen oxide ratio obtained by entering the exhaust gas temperature or a numerical variable related to this has been determined in the second processor unit. 2. The method according to claim 1, characterized in that a third processor unit is provided in which based on a relationship between a time change amount or rate of the exhaust gas temperature and the ammonia / nitrogen oxide ratio a characteristic factor for correcting the said ratio is provisionally derived and stored as a function of the amount of time change, and that the multiplication under Correction is carried out by the third processor unit. 909830/0867